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Move self to the explicit () parameter list (proposal #7016) (#7272)

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Implements proposal #7016: `self` moves from the deduced implicit list
(`fn F[self: Self]()`) to the front of the explicit list. Its type may
be written explicitly (`fn F(self: Self)`) or omitted, in which case it
defaults to `Self` (`fn F(self)`, `fn F(ref self)`); `self` in the
implicit list is rejected.

Throughout checking, `self` is modeled as the first explicit parameter.
Because a method is just a function whose first parameter is `self`, it
can also be called as an ordinary function with the receiver passed
explicitly (`Type.M(obj, ...)`), not only as `obj.M(...)`. A new
`SemIR::CallArgParamPatterns` helper chooses the parameters matched
against the explicit arguments, excluding a leading `self` only when it
is supplied as a method-call receiver; arity checking, conversion, and
generic deduction use it. The resulting SemIR and lowering are
unchanged: `self` is still `call_param0`, and witnesses, thunks, and
vtables are unaffected.

An omitted `self` type is parsed as a `SelfBindingPattern` node with no
type expression; checking synthesizes the `Self` type so it behaves
exactly like `self: Self`. However, the exact spelling used must match
between a forward declaration and a definition, following #3763's rules
around declaration matching.

Generated functions, thunks, and C++ interop import/export build `self`
as the first explicit parameter, and the `self`-type override (e.g.
Derived->Base for a virtual override) applies to the explicit `self`.
Placement is validated by new diagnostics: `SelfInImplicitParamList`,
`SelfNotFirstParam`, and `SelfOutsideParamList`. The benchmark source
generator and the documentation adopt the `(self)` shorthand; the
prelude, the examples, and the test data are migrated in the following
commits.

Assisted-by: Claude Code with Claude Opus 4.7

---------

Co-authored-by: josh11b <15258583+josh11b@users.noreply.github.com>
This commit is contained in:
Chandler Carruth
2026-06-10 15:30:43 +00:00
committed by GitHub
co-authored by josh11b
parent fb05da761f
commit 7871237c15
296 changed files with 6101 additions and 4899 deletions
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toolchain/check/testdata/function/definition/syntactic_merge.carbon
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@@ -56,7 +56,7 @@ library "[[@TEST_NAME]]";
class C {}
// CHECK:STDERR: fail_only_implicit_params.carbon:[[@LINE+8]]:8: error: implicit parameters of functions must be constant or `self` [ImplictParamMustBeConstant]
// CHECK:STDERR: fail_only_implicit_params.carbon:[[@LINE+8]]:8: error: implicit parameters of functions must be constant [ImplictParamMustBeConstant]
// CHECK:STDERR: fn Foo[a: C];
// CHECK:STDERR: ^~~~
// CHECK:STDERR:
@@ -65,7 +65,7 @@ class C {}
// CHECK:STDERR: ^~~~~~~~~~~~~
// CHECK:STDERR:
fn Foo[a: C];
// CHECK:STDERR: fail_only_implicit_params.carbon:[[@LINE+8]]:8: error: implicit parameters of functions must be constant or `self` [ImplictParamMustBeConstant]
// CHECK:STDERR: fail_only_implicit_params.carbon:[[@LINE+8]]:8: error: implicit parameters of functions must be constant [ImplictParamMustBeConstant]
// CHECK:STDERR: fn Foo[a: (C)] {}
// CHECK:STDERR: ^~~~~~
// CHECK:STDERR: